The analog and digital transmission methods used for data transmission on line and cables are exposed to disturbing influences, for example, electromagnetic fields, that modify the information to be transmitted. For example, one or more bits can be "turned around" during the transmission. Since international long-distance connections are especially susceptible to bit errors, international standardization committees (such as, for example, CCITT) have defined minimal rules for bit error rates. Optical transmission links are considerably less subject to external influences, so that the transmission reliability is extremely high in optical transmission links.
Every transmission link is generally composed of a total of two parts:
One part relates to the transmission of the information via a physical link between two nodes. These links are being increasingly realized with optical communications cables. PA0 Another part relates to the through-connection of the information in the respective nodes wherein communication systems are installed.
As a result of the employment of optical transmission systems, the potential noise influence has been decisively reduced in the transmission-oriented part. By comparison thereto, bit errors frequently occur during the throughconnection events in the switching networks of the communication systems.
Due to their central significance for the communication systems, the switching networks are usually executed in duplicate or in triplicate configurations. This makes it possible to through-connect the bit stream via a different switching network level given malfunction of a switching network level. This is particularly true for errors that lead to a total outage of a switching network level.
However, bit falsifications caused by aging of individual component parts, for example, are problematical during the through-connection of a bit stream. Such errors are difficult to analyze and eliminate since they have no influence on the function of the switching network as a switching unit.
Error recognition methods for recognizing and correcting bit errors in switching networks are realized, for example, in the form of an ongoing supervision. German Published Application 24 27 668 discloses such a method. Therein, an additional parity bit is attached to the information words per channel that form the information before the actual through-connection event in the switching network, whereby an information word is usually composed of 8 bits. What is generally understood by a parity bit is the binary check sum across the individual bits of the corresponding information word. After the through-connection event, a binary check sum of the through-connection word is again calculated and compared to the transmitted parity bit. When the transmitted parity bit and the newly calculated parity bit deviate from one another, then there is a transmission error that is stored in a corresponding table memory. Statements about the condition of the corresponding switching network levels can thus be made over certain time spans. These statements are of a statistical nature and the switching network level that had the lowest bit error rate in the past can thus be preferably employed for the through-connection of the bit streams.
What is problematical in this method, however, is that the falsification of an even number of bits of an information word is not recognized as a bit error since the parity formation implemented at the output side leads to the same result as the parity formation that ensued at the input side. Further, although statistical statements abut the quality of the through-connection of the information words can be made for recognized bit errors, the bit error remains uncorrected and is thus forwarded to the terminal subscriber. This procedure harbors the risk that additional bit errors will add up in further network nodes.